1. Field of the Invention
The present invention relates to an optical disc such as an optical disc and a magneto-optical disc onto/from which data is written or read out by an optical disc recording/reproduction apparatus.
2. Description of the Prior Art
An optical disc including a magneto-optical disc has been suggested for saving audio, video, or computer data. This optical disc, for example, has a recording section on a transparent substrate and data is recorded on this recording section.
For example, as shown in FIG. 1, a magneto-optical disc having a signal recording plane only on one side is formed from a disc-shaped transparent resin such as polycarbonate includes: a substrate 102 having a thickness of, for example, about 1.2 mm; a recording layer 103 formed on the substrate 102; and a protection layer 104 formed on the recording layer 103. This magneto-optical disc 101 is an optical disk normalized by the International Standard Organization (ISO) such as ISO/IEC 10089 and 13549 and enables data writing by forming the recording layer 103 from a magnetized film. The magneto-optical disc 101 has a clamp portion 105 at the center portion.
Moreover, an optical disc recording and/or reproduction apparatus enabled to write or read data onto/from the magneto-optical disc 101 includes: a clamp 106 for holding a hub (not depicted) provided in the clamp portion 105 of the mounted magneto-optical disk 101 and for rotating the magneto-optical disc 101; and an objective lens 107 for focusing a laser beam emitted from a light source (not depicted) on the recording layer 103. The objective lens 107 and the clamp 106 are arranged at positions matching the configuration of the magneto-optical disc mounted on the optical disc recording and/or reproduction apparatus.
For example, the objective lens 107 is supported in such a manner that it can move within a predetermined stroke for focus servo. The objective lens 107 is positioned at a neutral position (position X1 in FIG. 1) of the focus servo for the magneto-optical disc 101.
The optical disc recording and/or reproduction apparatus is configured for writing or reading onto/from the magneto-optical disc 101.
Here, the magneto-optical disc 101 can have increase data recording density by decreasing the thickness of the substrate as shown in FIG. 2. It should be noted that FIG. 1 and FIG. 2 show a single-sided type magneto-optical disc, but a double-sided magneto-optical disc can have further increased data recording density.
It has been desired to provide an optical disc recording and/or reproduction apparatus capable of recording or reproducing data onto from both of the magneto-optical disc 101 having a conventional substrate thickness of about 1.2 mm. and a magneto-optical disc 109 which is said to enable a higher recording density by reducing a substrate thickness. That is, an apparatus compatible for both of the discs is desired.
In this case, it is considered to design a compatible optical disc recording and/or reproducing apparatus to match with, for example, the conventional magneto-optical disc 101. For example, if the objective lens 107 is subjected to a focus servo when a magneto-optical disc is mounted, data can be recorded or reproduced onto/from both of the magneto-optical discs 101 and 109.
However, if it is impossible to obtain a stroke (a distance between X1 and X2 in FIG. 2) required for carrying out a focus servo to the magneto-optical disc 109, the optical disc recording and/or reproduction apparatus cannot carry out focusing.
Furthermore, if focus is obtained at a shift limit position of the objective lens 107 during a focus servo, a focus characteristic is deteriorated because the objective lens 107 is not positioned at a neutral position for carrying out an appropriate focusing.
Moreover, in order to provide a greater shift stroke for the objective lens 107, the optical disc recording and/or reproduction apparatus should have an additional function.
It is therefore an object of the present invention to provide an optical disc onto which focusing can be obtained with the same objective lens even if the substrate thickness varies.
The optical disc according to the present invention includes: a transparent substrate; a recording layer formed on the transparent substrate; and a clamp portion provided at a center portion of the transparent substrate; the clamp portion and the transparent substrate having surfaces different in height so that the recording layer plane is positioned in the vicinity of a focus position of the objective lens which is at a neutral position.
According to another aspect of the invention, the optical disc has a higher recording density than a normalized optical disc, and includes: a transparent substrate having a smaller thickness than the normalized optical disc; a recording layer formed on the transparent substrate; and a clamp portion which has a thickness identical to that of the normalized optical disc and is rotatably supported by a clamper provided in an optical disc recording and/or reproduction apparatus.
The optical disc having the aforementioned configuration is enabled to minimize the stroke of the objective lens provided in the optical disc recording and/or reproduction apparatus.